1. Field of the Invention
This invention relates to a relatively compact, even pocket size, microfocused x-ray tube having a protruding bright beam spot. It is especially adaptable for an x-ray stepper useful for dimensional control and alignment in the manufacture of microelectronic devices such as an integrated circuit (IC) or computer chip. It is also useful for medical imaging. The invention also relates to a microelectronics alignment, positioning or dimensional control process and system.
ICs are conventionally produced by a lithographic process which includes passing light through a mask onto the surface of a silicon wafer substrate coated with photosensitive chemicals to produce an etched chip.
Over the years, etched lines have become increasingly fine and circuits more complex, resulting in ICs having a greatly increased number of components equivalent to transistors, enabling the power of the computer chips and the electronic devices utilizing them to be substantially increased.
The etching process requires that the light passed through the mask must form sharp line images. This becomes increasingly difficult for very fine lines which require much higher line resolution. Technology using ordinary light has enabled production of ICs with a minimum width of about 0.35 .mu.m, and having components equivalent to about a few million transistors. The use of coherent light is limited to producing a line width or resolution of down to about 0.3 .mu.m at reasonable cost. Ordinary light waves at 0.5 .mu.m are too large to give sharp images at smaller line widths.
However, the semiconductor industry is moving toward 0.1 .mu.m line resolution and contemplates use of 12 or more masking levels. These requirements indicate an urgent need for improvements in critical dimension controls of mask-to-wafer alignment during wafer exposure, in absolute positioning during mask writing, and in metrology in general.
2. Description of Related Art
X rays have been investigated for such needs, due to their very short wave length. The generation of x rays for IC lithography or dimension and positioning control has been a problem. Ordinary sources such as medical x-ray tubes are not useful because of production of unduly strong x rays which penetrate the mask and wafer and do not enable production of sharp images without damage to the materials.
Soft (low energy) x rays have been used for lithography and positioning, but have been produced by bulky equipment which is both expensive and not optimal for working with the relatively small wafers from which computer chips are made. For example, synchrotrons have been used to produce soft x rays for lithography.
U.S. Pat. No. 3,984,680 issued to Henry I. Smith discloses an alignment system for aligning a mask and substrate to be exposed using soft x rays. Alignment marks with regions that transmit soft x rays and regions opaque to soft x rays are provided on the mask. Geometrically similar marks are provided on the substrate. The marks on the substrate contain materials which fluoresce when struck with x rays used for alignment, i.e., emit x rays. The emitted x rays are detected. Soft x rays from an x-ray source pass through the transparent regions and are absorbed by the opaque regions of the marks on the mask. The amount of x rays striking the x ray emitting marks on the substrate is dependent on the alignment or superposition of the mask marks and the substrate marks. The magnitude of the detected emitted x rays indicates the degree of superposition, and can be used as a basis for moving the mask or substrate into the desired registration. The system of this patent requires operating in a vacuum.
Another alignment system is described in U.S. Pat. No. 3,742,229 issued to Henry I. Smith et al. In this patent, soft x rays are used with a first registration means on a mask and a second registration means on a substrate.
U.S. Pat. No. 4,238,685 issued to Peter Tischer describes an alignment system employing displacement of x-ray beam source location with respect to a mask in fixed association with a semiconductor body.
U.S. Pat. No. 3,743,842 issued to Henry I. Smith et al. discloses a lithographic process using soft x rays.
U.S. Pat. No. 5,044,001 issued to Chia-Gee Wang discloses a method and a apparatus for investigating materials in which x rays are generated in a thin metal foil inside an evacuated x-ray tube, and a specimen outside the tube is exposed to the generated x rays. The disclosure of U.S. Pat. No. 5,044,001 is incorporated herein by reference.
U.S. Pat. No. 3,668,454 issued Jun. 6, 1972 to Shimura discloses a fine focus x-ray tube having a target at one end comprising a base with an opening containing a vertex and a hole or slit at said vertex across which a thin foil target is applied. An electron beam is converged by a converging coil and a deflecting coil. Said coils are arranged around the outside of said tube, said electron beam is focused in said vertex hole or slit and the position of the focus can be moved by means of the deflecting coil.